1. Field of the Invention
The present invention relates to a boat engine idling revolution number control device and method for appropriately controlling an engine idling revolution number of an internal combustion engine to be mounted on a boat.
2. Description of the Related Art
In the field of electronically-controlled internal combustion engines, the following technology of controlling an engine idling revolution number is conventionally well known. Specifically, when a predetermined idling condition is satisfied, an air amount to be supplied to the internal combustion engine (hereinafter also referred to as “engine”) is controlled so as to control an engine revolution number to a predetermined value. According to this kind of technology, the air amount is controlled by an air intake regulator valve in accordance with a difference between a target revolution number and an actual revolution number during the idling operation so that the difference is eliminated by feedback control.
Moreover, in accordance with a state in which the boat is running while decelerating or in accordance with a shift position state of a gear mechanism included in the engine, the engine revolution number is required to be controlled so as to be kept properly. In view of the requirements described above, an engine idling revolution number control device which can deal with specific uses of outboard motors has been proposed (see, for example, Japanese Patent No. 5289854).
The related art has, however, the following problems.
The related-art engine idling revolution number control device described above supplies the air amount in accordance with an engine load and performs the feedback control so as to eliminate the difference between the target revolution number and the actual revolution number during the idling operation of the engine so that the engine steadily operates at the target revolution number.
In the case of boats, however, the load on the engine greatly varies depending on the shift position state, the target revolution number during the idling, a boat speed during the idling, or the like. In particular, in the case where a decelerating operation is performed by closing a throttle valve to bring the engine into an idling state while the boat is running (moving forward with the shift position “forward”) at a high boat speed (for example, at 50 km/h or higher), the boat speed does not drop immediately. During the decelerating operation, a propeller is driven by a water stream. Therefore, the engine is driven by the propeller.
In the case described above, even when the engine is in the idling state, the engine revolution number becomes significantly higher than the target revolution number during the idling. If revolution number feedback is implemented in this state, the air amount to be supplied to the engine is excessively reduced by revolution number feedback correction in order to decrease the engine revolution number. If the shift position is put into neutral in this state, an engine driving force by the propeller cannot be obtained anymore. Moreover, the air amount for maintaining the idling state is insufficient. Thus, an engine stall occurs in some cases.
As described above, in order to avoid the engine stall due to feedback divergence caused by a disturbance such as the drive by the propeller, a revolution number feedback gain cannot be set higher. Therefore, it is conventionally difficult to achieve both stability of the engine idling revolution number control device and responsiveness during a transition such as during deceleration.
Moreover, the size and shape of the propeller to be mounted differ for each boat in many cases depending on purposes of use specific to the boats. Therefore, a running load (torque) when the boat moves forward or backward during idling varies depending on the type of propeller. Therefore, a difference is sometimes generated between an engine output torque determined based on a set value and a torque necessary to maintain the target revolution number. In this case, a deviation is generated in revolution number and therefore is absorbed by the revolution number feedback.
Moreover, when the throttle valve is brought into the idling state again and the shift position is put into neutral while the boat is running forward, the engine is not driven by the propeller. Therefore, the engine revolution number can be smoothly converged to the target revolution number. However, when the shift position is put into reverse while the speed of forward movement of the boat is high, the propeller is rotated in a forward direction (to move forward) by the water stream. Therefore, the engine rotates the propeller in an opposite direction (to move backward), and hence a propeller load on the engine is large. Thus, when the boat speed is high, the propeller load does not balance the output torque of the engine. Thus, there is a problem in that an engine stall is likely to occur.
Moreover, the following system has been proposed as the related art and has been put into practical use. Specifically, the system uses a shift-position detection sensor for detecting the shift position of a gear mechanism of the engine (neutral, forward, or reverse) to correct the air amount in accordance with the shift position so as to avoid the engine stall by the shifting operation for the deceleration. In the case of a small outboard motor, however, the shift position detection sensor is difficult to use in view of costs or mountability. Thus, a neutral switch (SW) capable of detecting only the shift position “forward or reverse” or “neutral” is commonly used.